1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming capable of effectively fixing a visible image on a recording medium.
2. Discussion of the Related Art
In a typical conventional image forming apparatus, a latent image formed on an image carrying member is developed with toner supplied from a developing device, and a visible toner image is formed on the image carrying member. The toner image formed on the image carrying member is then transferred to a recording medium by a transferring device and is fixed on the recording medium by a fixing device. Fixing members used in the fixing device include an endless fixing belt passing over at least two support members to rotate, and a press roller biasing the fixing belt with a predetermined pressure to form a nip, such as a fixing device that presses a recording medium by the heated fixing belt and the press roller to fix the toner image formed on the recording medium thereon with heat and pressure.
To improve the quality of an image formed by a color image forming apparatus, the fixing member can have a surface layer formed by an elastic layer. If the fixing member is formed of a rigid material and does not have a surface layer formed by the elastic layer, a surface of the fixing member contacting the recording medium (such as paper) does not fit microscopic concavities and convexities of a surface of the recording medium. As a result, the surface of the fixing member fails to closely contact the surface of the recording medium, resulting in deterioration in image quality (such as microscopic uneven glossiness of the image). This microscopic uneven glossiness may be referred to as “orange peel surface” problem. This problem is notably observed in an image formed by the color image forming apparatus, while the problem may not be particularly noticeable in an image formed by a monochrome image forming apparatus. In the color image forming apparatus, therefore, the surface of the fixing member should have elasticity to improve the quality of image.
Some conventional techniques attempt to solve the orange peel surface problem by focusing on hardness, such as JIS-A hardness, of the surface of the fixing member. One such fixing method is described in Japanese Laid Open patent publication No. 10-198201. In the fixing method in which low pressure is applied to a nip, however, the orange peel surface problem occurs. It is thus found that application of a certain level of pressure to the nip prevents the orange peel surface problem.
A fixing device performing image fixation by applying a certain level of pressure is disclosed, for example, in Japanese Laid-Open patent publication No. 2002-72752. FIG. 1 shows a schematic view of the fixing device as disclosed in the Japanese publication.
In FIG. 1, a fixing device 409 includes a fixing roller 401 and a press roller 402. The fixing roller 401 serves as a fixing member, and the press roller 402 serves as a press member. The fixing roller 401 includes a heater 405, a core metal 421, and a releasing layer 422. The heater 405 serves as a heating member. The core metal 421 is a base material having a rigid outer circumferential surface. The releasing layer 422 covers the outer circumferential surface of the core metal 421. Conversely, the press roller 402 includes a solid core metal 406, an elastic layer 423, and a polytetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) tube 424. The elastic layer 423 is formed of a silicone rubber and covers the outer circumferential surface of the core metal 406. The elastic layer 423 is covered by the PFA tube 424, which serves as a releasing layer. In the fixing device thus configured, the fixing roller 401 heated by the heater 405 and the press roller 402 press a recording medium with a surface pressure of about 0.2 N/mm2 to about 1.0 N/mm2, for example. Surface pressure is obtained by dividing applied load by area of the nip applied with the load. In the fixing device, the heater 405 sufficiently heats the fixing roller 401, generating a large surface pressure of about 0.2 N/mm2 or more, for example. Accordingly, the deterioration in image quality such as the uneven glossiness attributed to the microscopic concavities and convexities of the surface of the recording medium may be suppressed.
High pressure load needs to be applied to the fixing roller 401, however, to generate such a large surface pressure of about 0.2 N/mm2 or more for preventing the orange peel surface problem attributed to the microscopic concavities and convexities of the surface of the recording medium. Further, the nip needs to have a large nipping area required for performing the image fixation. To obtain the required large nipping area, an elastic layer having a sufficient thickness or sufficient flexibility is required. If the elastic layer having sufficient flexibility is used, the elastic layer laterally extends, preventing the surface pressure from increasing, even if high pressure load is applied on the elastic layer. Therefore, the elastic layer having sufficient flexibility is not preferred. Accordingly, to form the nipping area having the large width required for performing the image fixation as well as prevent the orange peel surface problem, an elastic layer having a sufficient thickness and a rigid core metal having a diameter or thickness that prevents bending of the fixing roller under the high pressure load can be used. If the thickness of the elastic layer is increased, and the diameter or thickness of the core metal is increased, an amount of heat required for heating the fixing roller increases. As a result, time required for increasing the temperature of the fixing member up to a predetermined degree (referred to as start-up time) is increased. To reduce the start-up time, the temperature may be kept to a certain level by using residual heat. This attempt using the residual heat, however, is not preferable from a viewpoint of energy reduction.
Japanese Laid-Open patent publication No. 08-076620, discloses another fixing method referred to as an on-demand fixing. According to the method, a heat source is provided on the inside surface of the fixing belt forming the nip, and the nip is directly heated on demand. Accordingly, the start-up time is reduced. Further, the method requires no residual heat, thus saving energy.
However, due to the configuration in which the heat source is provided at the nip, it is difficult to apply sufficient surface pressure for preventing influence of the microscopic concavities and convexities of the recording medium surface. To prevent the deterioration in image quality such as the uneven glossiness attributed to the microscopic concavities and convexities of the recording medium surface while applying low pressure load to the fixing member, the fixing member should have a flexible surface layer. If a thick rubber layer is used to form the flexible surface layer, however, the amount of the heat required for heating the fixing member increases. As a result, the start-up time increases, making it difficult to perform the on-demand fixing method.
As described above, it is difficult to provide a fixing device capable of both reducing the start-up time and energy consumption and performing high-quality image fixation not affected by the microscopic concavities and convexities of the surface of the recording medium.